You don't have to be boyfriend and girlfriend to get along. Is she interested in you, is she professed, or is she just ambiguous? How long have they been apart? Generally, the one who is dumped will have a period of psychological depression. If she comes out so soon, she is either a playboy or she may really be using you. People's hearts and stomachs are separate. In short, you should be vigilant. Just don't have sex with that woman in a hurry, lest she use it to get back at your roommate.

If you mind them having sex, it means that you haven't fully accepted her. Let it be.